The invention concerns an apparatus for the treatment of biological tissue, in particular living tissue with laser radiation.
The therapeutic laser application to the eyeground, in particular in the case of retinal diseases, to achieve photocoagulation at the retina, is known. Lasers whose pulsed treatment radiation is in the green wavelength range are primarily used as treatment lasers in photocoagulation. That radiation is particularly strongly absorbed in the fundus layers of the eye. Solid state lasers, for example the frequency-doubled Nd:YAG laser of a wavelength of 532 nm are frequently used. Argon ion lasers at 514 nm are frequently also employed. The laser beams which are used in that case produce spot sizes of between 50 and 500 μm in the target tissue. The laser power levels can be up to several hundred mW, wherein irradiation durations of between 50 ms and 500 ms are employed. In the case of diabetic retinopathy panretinal photocoagulation, with the macula being cut out, is used, in which the treatment is performed over a large area with some hundred to over a thousand coagulation spots. In addition photocoagulation is used in relation to retinal holes and retina detachments. In that case a join between the retina and the eye background is produced in the edge regions of the retinal damage by scarring.
Hitherto the treatment parameters have been set on the basis of empirical values for dosing purposes. Because of different pigmentations in the eye the temperatures produced in the photocoagulation operation however may fluctuate to a greater or lesser degree.
EP 1 279 385 A1 discloses determining the temperature in the treatment of biological tissue in particular at the eyeground by means of laser radiation. In that case during the respective pulses of the pulsed treatment radiation additional radiation pulses of shorter pulse duration and lower energy level than in the treatment radiation are directed on to the target tissue. Tissue expansion and contraction phenomena which occur in that case produce bipolar pressure waves which are detected. Those measured pressure transients are used to ascertain the corresponding temperature values during the radiation treatment, with the aid of the Grüneisen calibration curve and a calibration temperature.
It is also known (DE 199 16 653 A1) to use the pressure transients generated in the target tissue as a measurement value for the optical properties of the target tissue in order thereby to control the further treatment procedure by comparison with especially ascertained characteristic curves in respect of the configuration of the changes in the optical properties during the therapy in fully automated on-line and computer-aided mode.
In photocoagulation however the tissue properties are modified at incipient denaturing of the target tissue, whereby the pressure amplitudes induced by the measurement radiations are also influenced.
The object of the invention is to provide an apparatus of the kind set forth in the opening part of this specification, in which the control of the treatment radiation is improved.
According to the invention that object is attained by an apparatus having the features of claim 1. The appendant claims recite advantageous developments of the invention.
In the invention the apparatus includes a device for generating a pulsed treatment radiation to be directed on to a target tissue, preferably a laser radiation. There is also a detector device for detecting pressure amplitudes coming from the target tissue. Those pressure amplitudes can be induced by the treatment radiation, in which case the treatment radiation is of a frequency of at least 100 Hz. It is also possible to additionally provide a measurement laser device for generating an additional pulsed measurement radiation directed on to the target tissue, of a lower energy level and a shorter pulse duration than in the case of the treatment radiation. In that case the detector device is suitable for detecting the pressure amplitudes induced by the measurement radiation and coming from the target tissue. There is also an evaluation device for evaluation of the pressure amplitudes detected by the detector device and a control device for controlling the treatment radiation in dependence on the evaluated pressure amplitudes.
In the treatment of the target tissue with the treatment radiation, in particular laser radiation, there is still no change in the tissue during a first, heating-up time interval Δt1 in the target tissue. The duration of that first time interval can be for example 20-50 ms.
During the treatment with the treatment radiation, radiation is preferably effected with additional pulsed laser radiation, as is known from EP 1 279 385 A1. The pressure amplitudes which occur in that case increase with the time t during the interval Δt1 on average in accordance with a function f(t).
The function m(t) represents the basic curvature characteristic of the temporal variation in the mean pressure amplitudes during the tissue heating-up phase in the absence of changes in the tissue. The function m(t) is known (Jochen Kandulla, Ralf Brinkmann, ‘Nicht-invasive Echtzeit-Temperaturbestimmung während Laserbehandlungen an der Netzhaut des Auges’: Photonic 2/2007, 42-46), it is based on the error function. It can be well approximated over short time intervals with different simpler functions. The function m(t) can be stored in a memory of the evaluation device or a memory connected to the evaluation device.
The fit factor a in accordance with the equation (f(t)=a*m(t) is unknown prior to measurement and is generally different in particular for each radiation location. The fit factor a depends on the probe laser energy and pigmentation of the area which is just being irradiated, but equally also on the propagation of sound in the eye, the acoustic impedance jump at the retina, the acoustic transducer geometry and sensitivity, signal amplification and so forth.
At each radiation location the averaged variation f(t) of the pressure amplitudes measured in the time interval Δt1 is fitted with the fit factor a which occurs in that case in accordance with the fit condition f(t)=a*m(t). Accordingly a function a*m(t) is also available in a time region Δt2 following the first time interval.
In Δt2, at each moment in time t, the relationship of the currently prevailing measurement data function f(t) which reproduces the averaged variation in the measured pressure amplitudes and the function a*m(t) is formed. By virtue of the measurement data noise, a mean value of the currently prevailing measurement values f(t) (for example from 10 measurement values) can advantageously be related to the function a*m(t), for example as V(t)=f(t)/[a*m(t)].
For a change in the tissue and in particular tissue coagulation the evaluation device is so designed that it establishes whether and when a given predetermined deviation V* in the current measurement value f(t) from the function occurs in the second time interval Δt2 following the first time interval (for example 20%, that is to say for example V*=0.8). According to the invention establishing such a significant deviation during the time interval Δt2 marks that tissue changes have occurred shortly before. Any continuation of the previous radiation would certainly produce even more severe tissue damage.
In particular the speed of implementation of tissue denaturing when irradiation is continued can be estimated from the moment in time ti at which the detected deviation V* occurs (that is to say V(ti)=V*). The radiation parameters of the treatment radiation for the third time interval Δt3 which follows the second time interval are established from the moment in time ti. They are afforded from previously experimentally obtained data. The experimentally ascertained data can be stored in a memory of the evaluation device or in a memory connected to the evaluation device.
The control circuit serving to control the treatment radiation can be adapted to control the duration and/or the power of the respective pulse of the treatment radiation. With the radiation power remaining the same the duration of Δt3 is determined from the moment in time ti or the duration of Δt2, wherein for example the duration of Δt3 can be selected to be proportional to the duration of Δt2. The shorter Δt2 is, the correspondingly shorter is Δt3.
The evaluation device is preferably in the form of a computer-aided evaluation device which includes corresponding memories for the function m(t) and the experimentally ascertained data required for control of the treatment radiation, in particular in the third time interval. This can involve data in respect of the processing time still to be applied and/or the power to be applied in respect of the treatment radiation. The invention makes it possible to use the measured pressure transients for control of the treatment radiation. In particular there is no need for calibration or standardisation to a temperature or other reference values.